Superconducting cable

ABSTRACT

Superconducting cable ( 1 ) comprising:  
     a) a layer ( 20 ) of tapes comprising superconducting material,  
     b) a tubular element ( 6 ) for supporting said layer ( 20 ) of tapes comprising superconducting material,  
     c) a cooling circuit, adapted to cool the superconducting material to a working temperature not higher than its critical temperature,  
     characterized in that said tubular element ( 6 ) is composite and comprises a predetermined amount of a first material having a first thermal expansion coefficient and a second material having a thermal expansion coefficient higher than that of said first material, said thermal expansion coefficients and said amounts of said first and second material being predetermined in such a way that said tubular element has an overall thermal shrinkage between the room temperature and said working temperature of the cable such as to cause a deformation of said tapes comprising superconducting material lower than the critical deformation of the same tapes.

FIELD OF INVENTION

[0001] In a general aspect thereof, the present invention relates to acable to be used to transmit electric current in conditions of so-calledsuperconductivity, i.e. in conditions of almost null electricresistance.

[0002] More particularly, the invention relates to a superconductingcable comprising:

[0003] a) a layer of tapes comprising superconducting material,

[0004] b) a tubular element for supporting said layer of tapescomprising superconducting material

[0005] c) a cooling circuit adapted to cool the superconducting materialto a working temperature non higher than its critical temperature.

[0006] In the following description and the subsequent claims, the term:superconducting material, indicates a material, such as for instancespecial niobium-titanium alloys or ceramics based on mixed oxides ofcopper, barium and yttrium, or of bismuth, lead, strontium, calcium,copper, thallium and mercury, comprising a superconducting phase havinga substantially null resistivity under a given temperature, defined ascritical temperature (in the following also shortly referred to as Tc).

[0007] The term: cable for high power, indicates a cable to be used fortransmitting current quantities generally exceeding 3,000 A, such thatthe induced magnetic field starts to reduce the value of the maximumcurrent density achievable in superconductivity conditions.

[0008] The term: superconductor cable indicates in the following anyelement capable of transmitting electric current in superconductivityconditions, such as for example tapes of superconducting material woundonto a supporting core.

[0009] The superconducting cables comprise a structural elementconsisting of the supporting tubular element of the superconductingmaterial.

[0010] Patent application EP 97202433.5 in the name of the Applicantdiscloses a supporting tubular element entirely consisting of a tubemade of polymeric material, typically polytetrafluoroethylene orpolyamide.

[0011] The same patent application EP 97202433.5 also discloses asupporting tubular element made of metallic material such as steel,copper or aluminum.

[0012] The superconducting cable is installed at room temperature, aswell as the electrical (to the terminals) and hydraulic connections(attached to the cooling circuits of the cable).

[0013] After the installation the cable is brought to its workingtemperature by means of the cooling liquid. During such cooling eachcomponent of the cable is submitted to mechanical stresses of thermalnature, according to the thermal coefficients of the constitutingmaterials.

[0014] In particular mechanical stresses are generated in the layers ofsuperconducting materials and at the terminals connected to the ends ofthe cable.

[0015] The Applicant has noticed that the supporting element must notonly offer a satisfactory mechanical support to the layer or layers ofsuperconducting material, but also at the same time perform a number ofadditional functions not less important for the good operation of thecable.

[0016] More particularly, the supporting element should:

[0017] i) ensure that during cooling of the cable no internal stressesare generated within the superconducting material nor at the ends of thecable;

[0018] ii) ensure the mechanical stability of the cable, that is to say,the cable can be bent according to bending radiuses compatible with thediameters of the reels onto which the cable is wound for its transport;

[0019] iii) contribute to the mechanical resistance of the cable duringthe installation; and

[0020] iv) substantially contribute to the cryostability of the cable incase of short circuit, this term indicating both keeping thesuperconducting material below its critical temperature and keeping thecooling fluid in liquid state.

[0021] The Applicant has found that the use of a substantially compositesupporting tubular element allows to reduce the stresses imparted to thesuperconducting material both in radial direction and along alongitudinal direction, while ensuring at the same time a sufficientamount of metallic material for ensuring the cryostability of the cable.

[0022] According to a first aspect the invention relates to asuperconducting cable of the above indicated type, which ischaracterized in that said tubular element is composite and comprises apredetermined amount of a first material having a first thermalexpansion coefficient and a second material having a thermal expansioncoefficient higher than that of said first material, said thermalexpansion coefficients and said amounts of said first and secondmaterial being predetermined in such a way that said tubular element hasan overall thermal shrinkage between the room temperature and saidworking temperature of the cable such as to cause a deformation of saidtapes comprising superconducting material lower than the criticaldeformation of the same tapes.

[0023] In a second aspect thereof, the invention relates to asuperconducting element characterized in that said tubular element issubstantially composite and comprises a predetermined amount of a firstmetallic material in electrical contact with the layer ofsuperconducting material and at least one second polymeric materialassociated to said first material.

[0024] According to a third aspect of the invention, a method forlimiting the tensile stresses along a longitudinal direction imparted toopposite fixing terminals of a superconducting cable of the type withclamped heads as a consequence of cooling is provided, the cablecomprising at least one layer of superconducting material, which ischaracterized by providing in the cable a composite tubular element forsupporting the layer of superconducting material.

[0025] In the following description and in the subsequent claims, theterm: superconducting cable of the type with clamped heads, indicates acable whose opposite ends are mechanically constrained to respectivefixing terminals in such a way that no substantial relative sliding inaxial direction between tapes and supports and with respect to theterminal themselves takes place.

[0026] Advantageously, the aforesaid composite supporting tubularelement is able not only to adequately support the superconductingmaterial, but also to limit the stresses induced along a longitudinaldirection in the layer of superconducting material and in the terminalsconnected to the ends of the cable and to provide at the same time anamount of metal in electrical connection with the superconductingmaterial, capable to substantially contribute to the cryostability ofthe cable during the short circuit transient.

[0027] In particular, it has been found that such composite supportingtubular element, thanks to the presence of the above indicated secondmaterial having a higher thermal expansion coefficient, has an overallthermal expansion coefficient equal to or higher than that of thesuperconducting material, and therefore during the cooling step of thecable is able to shrink in radial direction to a greater extent withrespect to entirely metallic supports, or, anyway, to an extent such asnot to cause unacceptable deformations in the tapes.

[0028] In this way, the composite support according to the inventionallows a greater shrinkage thereof along a longitudinal direction and,hence, allows to reduce the stresses along a longitudinal directionwithin the superconducting material due to the so-called constrainedshrinking by the clamped heads.

[0029] Additionally, the use of a composite supporting tubular elementadvantageously allows to reduce in a substantial way also the stressesexerted along a longitudinal direction by the ends of thesuperconducting cable on the terminals with respect to the tubularelements entirely made of metal whenever the second material of thecomposite supporting tubular element also has a Young's modulus (E)lower than that of the first metallic material.

[0030] The longitudinal stresses to which the supporting element of thecable is submitted in operation, in fact, are proportional to theproduct of the thermal expansion coefficient and the respective Young'smodulus (E) of the material which constitutes the supporting tubularelement.

[0031] In contrast to the tubular element entirely made of polymericmaterial, furthermore, the composite tubular element of the inventionallows to have in any case an amount of normal conductor in electricalconnection with the superconducting material, which is sufficient forensuring the cryostability of the cable during the short circuittransient.

[0032] For the purposes of the invention, the first metallic material ofthe composite supporting element is a metal preferably having aresistivity at 77 K<5*10-9 Ωm, a specific heat at 77 K>106 J/m3K and aheat conductivity at 77 K>5 W/mK.

[0033] In particular, the first metallic material of the compositesupporting element is selected from the group comprising: copper,aluminum and alloys thereof.

[0034] Preferably, the aforesaid second material is a non metallicmaterial and has a thermal expansion coefficient higher than 17*10-6°C.-1, preferably higher than 20*10-6° C.-1, and still more preferablycomprised between 40 and 60*10-6° C.- 1.

[0035] In a preferred embodiment, the aforesaid second non metallicmaterial is a plastics material.

[0036] For the purposes of the invention, the plastics material ispreferably selected from the group comprising: polyamide, such as forexample nylon, polytetrafluoroethylene (PTFE), polyethylene.

[0037] The values of the percent thermal shrinkage (ε) between the roomtemperature and 77K and of the Young's modulus (E) at 77K of somematerials provided for use when manufacturing the composite supportingelement according to the invention, are indicated in the followingtable. Material ε (%) E (GPa) Cu 0.30 100 Al 0.39 77 Ag 0.36 100 PTFE2.00 5

[0038] In an advantageous embodiment, the aforesaid first and secondmaterials are formed as adjacent annular sectors. Such design allows, inparticular, to facilitate the step of manufacturing the compositetubular element.

[0039] For the purposes of the invention, the number of sectors of saidfirst and second material and the arrangement of such sectors may beeasily determined by a man skilled in the art on the basis of theconstruction requirements of the cable. For example, if a particularlyhigh thermal shrinkage is required, the metallic portion may be reduced,for example down to 10% or less, while if greater stiffness or specificelectrical characteristics are required, the polymeric portion may beconsequently reduced.

[0040] Preferably, the number of sectors for manufacturing a compositesupporting tubular element is comprised between 3 and 50. In a preferredembodiment, such number is chosen as a function of the outer diameter ofthe composite supporting tubular element and of the thickness of thesectors in such a way that the ratio “K” between the thickness “s” ofthe sector and its width “l” is comprised between 0,4 and 0,7.

[0041] Preferably, the sectors of said first and second material arealternately arranged one after the other. Such arrangement allows infact to make a supporting tubular element having mechanicalcharacteristics as homogeneous as possible which allow to ensure both asatisfactory dynamic stability of the stranding machine used formanufacturing the supporting tubular element, and the mechanicalcongruence of the composite supporting tubular element as a whole duringthe cooling of the cable.

[0042] Preferably, the annular sectors of said first and second materialare spirally wound with a winding angle comprised between 5° and 50°. Insuch a way, it is possible to ensure a satisfactory and lasting clampingbetween adjacent sectors.

[0043] According to a further embodiment, the composite supportingtubular element of the superconducting material may comprise an innertubular element essentially consisting of said second material ontowhich thin foils or wires essentially consisting of said first metallicmaterial are wound.

[0044] Preferably the phase conductor comprises at least onesuperconducting tape wherein said layer of superconducting material isincorporated within a metallic coating.

[0045] Advantageously, the cable of the invention comprises a pluralityof superconducting tapes spirally wound on the surface of the supportingtubular element according to a winding angle comprised between 5° and60°, and preferably between 10° and 40°. In such a way, it isadvantageously possible to further reduce possible mechanical stressesgenerated inside each of the aforesaid tapes.

[0046] According to an alternative embodiment, the phase conductorcomprises at least one reinforcing foil of metallic material coupled,preferably in a substantially irreversible way, to the metallic coatingof the superconducting tape and in electrical connection with thesuperconducting material.

[0047] In this way, during the short circuit transient, the overcurrentis split up between the metallic material of the tape, the metallicmaterial of the supporting tubular element and the reinforcing foil,electrically connected in parallel to the superconducting material andconstituting a resistive type conductor, by-passing the latter. At theend of the short circuit transient, the current may be transported againby the superconducting material in superconductivity conditions.

[0048] In particular, in the conductive element the electricalconnection of the metallic material of the tape with the metallicmaterial of the supporting tubular element on the one hand, and with thereinforcing foil on the other hand, is made either placing the aforesaidmaterials in direct contact with one another or interposing between themconductive elements known per se.

[0049] Preferably, the reinforcing foil has a thickness not higher thanhalf of the thickness of the metallic coating and advantageouslycontributes to increase the resistance of the conductive element of thecable at the various mechanical or thermal stresses, imparted theretoduring installation or use.

[0050] Still more preferably, such thickness is comprised between 0.03and 0.08 mm.

[0051] In a preferred embodiment of the invention, the resistance of theconductive element of the cable to the various stresses imparted theretomay be advantageously further increased submitting the superconductingmaterial to a predetermined prestress degree along a longitudinaldirection.

[0052] Such a prestress is preferably obtained by coupling thereinforcing foil to the coating material of the tape of superconductingmaterial, while simultaneously applying to the foil a tensile stresssubstantially oriented along a longitudinal direction.

[0053] Advantageously, it has been found that such a prestress of thesuperconducting material is able to partially compensate the tensileeffect applied on the superconducting material in the clamped headsarrangement of the cable when the latter is cooled from room temperatureto the temperature of the cooling fluid.

[0054] Preferably a conductive element provided with reinforced tapes ofthe above mentioned type is obtained by applying a tensile stresscomprised between 3.4*107 Pa (3.5 kg/mm²) and 34.3*107 Pa (35 kg/mm²) tothe reinforcing foil by means of apparatuses known per se, such as forexample by means of two coils, one for winding and the other forunwinding, of which one is suitably braked.

[0055] Due to such tensile stress, the superconducting material of thereinforced tapes so obtained has a % prestress degree along alongitudinal direction or “γ”, defined as follows:

γ=[(Li−Lf)⁻ /Li]*100

[0056] wherein:

[0057] Li=initial length of the tape;

[0058] Lf=final length of the tape after prestress;

[0059] comprised between 0.05 and 0.2%.

[0060] Preferably, the phase conductor comprises two reinforcing foilsmade of metallic material coupled to opposite faces of the metalliccoating.

[0061] Preferably, the reinforcing foil and the metallic coating arereciprocally coupled in a substantially irreversible way by means ofwelding or brazing and in such a way as to ensure that the desiredprestress of the superconducting material be maintained once thecoupling is made. Advantageously, the desired electrical contact betweenthe reinforcing foil and the metallic coating of the superconductingmaterial is automatically ensured in case of coupling by means ofwelding or brazing.

[0062] Preferably, the reinforcing foil or foils and the metalliccoating of said at least one superconducting tape consist of a metalselected from the group comprising: copper, aluminum, silver, magnesium,nickel, bronze, stainless steel, beryllium and alloys thereof.

[0063] Still more preferably, the reinforcing foil or foils coupled tothe metallic coating of the superconducting tape or tapes consist of ametal selected from the group comprising: stainless steel, preferablyamagnetic, bronze, beryllium, aluminum, and alloys thereof, whereas themetallic coating consists of a metal selected from the group comprising:silver, magnesium, aluminum, nickel, and alloys thereof.

[0064] The superconducting cable of the invention may be both a coaxialand a non-coaxial cable.

[0065] In the following description and in the subsequent claims, theterm: coaxial cable, indicates a cable comprising a supporting tubularelement, a phase conductor coaxially surrounding the supporting tubularelement, a layer of dielectric material external to the phase conductorand a return conductor supported by the layer of dielectric material andcoaxial to the phase conductor.

[0066] For the purposes of the invention, inside the return conductor acurrent flows which is equal and opposite to that circulating inside thephase conductor, so as to generate a magnetic field equal and oppositeto that generated by the current circulating in the phase conductor, soas to confine the magnetic field in the portion of the cable comprisedbetween the two conductors and reduce the presence of dissipativecurrents in the cable portions externally supported with respect to thereturn conductor.

[0067] Preferably, the return conductor comprises at least onesuperconducting tape including a layer of superconducting materialincorporated within a metallic coating and a predetermined amount ofmetallic material (stabilizing metal) in electrical contact with themetallic coating and having the function of allowing the stabilizationof the superconducting material in short circuit conditions.

[0068] Preferably, besides, the overall amount of the stabilizing metalis determined by applying the same criterion of full and adiabaticstability which is applied for the phase conductor and which will bereported in the following description.

[0069] Preferably, the stabilizing metal is split up in a plurality ofstraps or tapes, having a thickness comprised between 0.1 and 5 mm, indirect contact with the metallic coating of the superconducting tape,for example wound thereon.

[0070] In an alternative embodiment, the return conductor may compriseat least one metallic reinforcing foil coupled, preferably in asubstantially irreversible way, to the metallic coating of thesuperconducting material and interposed between the latter and thestabilizing metal.

[0071] Similarly to what happens to the phase conductor, if the returnconductor looses its superconducting capacities during the short circuittransient and the current passes through the stabilizing metallicmaterial, the reinforcing foil (if present) and the metallic coating ofthe tapes (if present), to flow back in the superconducting material atthe end of the short circuit.

[0072] Conveniently, the stabilizing metal of the return conductor,externally placed with respect to the superconducting tapes, may besplit up in straps or wires, for example of copper or other suitablemetal, associated to the superconducting tapes and, as such, also beingspirally wound as the same tapes.

[0073] Preferably, the superconducting cable of the invention is cooledby means of a suitable pressurized and undercooled cooling fluid, insuch a way as to ensure the heat exchange necessary for the operation ofthe cable and ensure that a temperature suitably lower than the criticaltemperature of the superconducting material is maintained, also for highlengths of the cable.

[0074] During its flowpath, in fact, the cooling fluid is simultaneouslysubmitted both to an increasing heating, as a result of the heatabsorbed by the elements which constitute the cable, and to anincreasing loss of pressure, due to the hydraulic losses while passingthrough the cable and to the more or less turbulent flow of the coolingfluid itself.

[0075] The choice of the working conditions of the cable is thereforemade taking such phenomena into account. In particular, workingconditions are preferred which maintain the cooling fluid far away fromthe temperature and pressure values of its own curve of saturation. Suchworking conditions are comprised inside the so called “working window”which delimits a portion in the state diagram of the cooling fluidinside which safety conditions exist with respect to the need of coolingthe superconducting material below its critical temperature whilemaintaining the cooling fluid in liquid state.

[0076] Advantageously, the use of pressurized and undercooled coolingfluid allows, furthermore, to reduce the amount of metallic materialemployed as stabilizing metal.

[0077] Preferably, the superconducting material is of the so called“high temperature” type (Tc of about 110 K) and is cooled to atemperature comprised between about 63 K and 90 K.

[0078] Such cooling is preferably achieved using liquid nitrogen ascooling fluid at a working pressure comprised between 10 and 20 bar.

[0079] According to the invention, the embodiments of the previouslydescribed superconducting cable may be various. In particular and asillustrated above, the cable of the invention may be coaxial ornon-coaxial, the phase or the three existing phases may be monoelementor multielement, the electrical insulation may be both in cryogenicenvironment (cold dielectric) or at room temperature (warm dielectric)the thermal insulation may be made on each single phase or on threejoined phases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Further characteristics and advantages of the present inventionwill appear more clearly from the following detailed description of somepreferred embodiments, made hereinbelow, by way of non-limitativeindication, with reference to the attached drawings. In the drawings:

[0081]FIG. 1 shows a perspective view in partial cross-section section,of a coaxial multielement triphase superconducting cable according to afirst embodiment of the present invention;

[0082]FIG. 2 shows a perspective view in an enlarged scale and inpartial cross-section, of an element of the coaxial cable of precedingFIG. 1;

[0083]FIG. 3 shows a perspective view in an enlarged scale and inpartial cross-section, of a second embodiment of an element of thecoaxial cable of preceding FIG. 1, wherein both the phase conductor andthe return conductor are provided with a reinforcing foil;

[0084]FIG. 4 shows a perspective view in an enlarged scale and inpartial cross-section, of a monophase, multielement non-coaxialsuperconducting cable according to a further embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0085] With reference to FIG. 1, a coaxial triphase superconductingcable 1 according to the present invention comprises a superconductingcore, globally indicated by 2, comprising a plurality of conductiveelements 3, indicated by 3 a, 3 b, 3 c for each phase, housed—preferablyloosely—within a tubular containing shell 9, made e.g. of metal, such assteel, aluminum and the like.

[0086] Each of the conductive elements 3 comprises in turn a couple ofcoaxial conductors, respectively phase conductors 4 and returnconductors 5, each including at least one layer of superconductingmaterial, as will appear more clearly in the following.

[0087] The coaxial phase conductors 4 and neutral conductors 5 areelectrically insulated from one another by interposing a layer 8 ofdielectric material, onto which the return conductor 5 is directlysupported.

[0088] The cable 1 also comprises suitable means for circulating acooling fluid adapted to cool the superconducting core 2 to atemperature adequately lower than the critical temperature of the chosensuperconducting material, which in the cable of FIG. 1 is of theso-called high-temperature type.

[0089] The aforementioned means comprises suitable pumping means, knownper se and therefore not shown, supplying a suitable cooling fluid, forinstance liquid nitrogen at a temperature typically of from 65 to 90 K,both within each of the conductive elements 3 and within the intersticesbetween such elements and the tubular shell 9.

[0090] In order to reduce as much as possible the thermal dissipationstowards the external environment, the superconducting core 2 is enclosedin a containing structure or cryostat 10, comprising a thermalinsulation, formed for instance by a plurality of superimposed layers,and at least one protection sheath.

[0091] A cryostat known in the art is described, for instance, in anarticle of IEEE TRANSACTIONS ON POWER DELIVERY, Vol. 7, nr. 4, October1992, pp. 1745-1753.

[0092] More particularly, in the example shown, the cryostat 10comprises a layer 11 of insulating material, formed, for instance, byseveral surface-metallilzed tapes (for instance some tens) made of apolyester resin, known in the art as “thermal superinsulator”, looselywound, with the possible help of interposed spacers 13.

[0093] Such tapes are housed in an annular hollow space 12, delimited bya tubular element 14, in which a vacuum in the order of 10-2 N/m2 ismaintained by means of known apparatuses.

[0094] The supporting tubular element 14 made of metal is capable ofproviding the annular hollow space 12 with the desired fluid-tightcharacteristics, and is covered by an external sheath 15, for instancemade of polyethylene.

[0095] Preferably, the supporting tubular metal element 14 is formed bya tape bent in tubular form and longitudinally welded, made of steel,copper, aluminum or the like, or by an extruded tube or the like.

[0096] If the flexibility requirements of the cable so suggest, element14 may be corrugated.

[0097] In addition to the described elements, cable traction elementsmay also be present, axially or peripherally located according to theconstruction and use requirements of the same, to ensure the limitationof the mechanical stresses applied to the superconducting elements 3;such traction elements, not shown, may be formed, according totechniques well known in the art, by peripherally arranged metalreinforcements, for instance by roped steel wires, or by one or moreaxial metal ropes, or by reinforcements made of dielectric material, forinstance aramidic fibers.

[0098] Several superconducting elements are present for each phase, inparticular, as shown by way of example in FIG. 1, each phase (a, b, c)comprises two superconducting elements, respectively indicated by thesubscripts 1, 2 for each of the three illustrated superconductingelements 3 a, 3 b, 3 c, so that the current of each phase is split upamong several conductors (two in the example shown).

[0099] In FIG. 2 one of the conductive elements 3 of the coaxialsuperconducting cable 1 of the preceding FIG. 1 is shown in perspectiveand enlarged scale.

[0100] To make the description easier, in the present FIG. 2 and thefollowing FIGS. 3 and 4, the elements of the cable structurally orfunctionally equivalent to those previously described with reference toFIG. 1 will be indicated by the same reference numbers and will be nolonger discussed.

[0101] The conductor element 3 a 1, shown in FIG. 2, comprises acomposite tubular element 6 including a plurality of annular sectors 16,17 respectively made of polymeric material, for examplepolytetrafluoroethylene, and of metallic material, for example copper,alternately arranged one after the other and spirally wound.

[0102] Each of the coaxial phase conductors 4 and return conductor 5comprises a plurality of superconducting tapes 18 a and 18 b,respectively, spirally wound on the composite tubular element 6 and ontothe layer 8 of dielectric material, respectively. Each of suchsuperconducting tapes 18 a, 18 b comprises a layer of superconductingmaterial 20 enclosed within a metallic coating 19.

[0103] The return conductor 5 further comprises a plurality of copperstraps 7 acting as stabilizing metal, in electrical contact with themetallic coating 19 of the superconducting tapes 18 b onto which theyare wound in a way known per se.

[0104] In the further embodiment of the conductive elements 3 of thecable 1 shown in FIG. 3, the coaxial phase conductor 4 and returnconductor 5 further include a plurality of metallic reinforcing foils 21coupled in a substantially irreversible way, for example by means ofbrazing, to the metallic coating 19 of the superconducting tapes 18 a,18 b.

[0105] Preferably, the reinforcing foils 21 of the phase conductor 4 arecoupled to a radially inner face of the metallic coating 19, so as to beinterposed between the composite tubular element 6 and the tapes 18 aand act in this way as elements of mechanical protection for the latter.

[0106] In such way, the foils 21 are in electrical contact both with themetallic coating 19 of the superconducting tapes 18 a and with theannular sectors 17 of metallic material of the composite tubular element6.

[0107] Preferably, the reinforcing foils 21 of the return conductor 5are arranged in a mirror-like fashion with respect to those of the phaseconductor 4, that is, they are coupled to a radially outer face of themetallic coating 19 of the tapes 18 b, so as to be interposed betweenthe copper straps 7 and the tapes and act in this way as elements ofmechanical protection for the latter.

[0108] In this way, the foils 21 of the return superconductor 5 are inelectrical contact both with the metallic coating 19 of thesuperconducting tapes 18 b and with the stabilizing metal (copper straps7).

[0109] Advantageously, furthermore, the reinforcing foils 21 of thecoaxial phase conductor 4 and return conductor 5 contribute both toensure the cryostability of cable 1 in case of short circuit and toadequately reduce the tensile stresses applied to the terminals of cable1 when the foils are coupled to the superconducting tapes 18 a, 18 b insuch a way as to impart to the superconducting material a predeterminedprestress degree.

[0110] In the embodiment shown in FIG. 4, that illustrates a non-coaxialand monophase superconducting cable 1, instead, the conductive elements3I, 3II, . . . , 3VII, only comprise the phase conductor 4 which, inthis case, includes superconducting tapes 18 spirally wound on thecomposite supporting tubular element 6.

[0111] If necessary, the cryostat 10 may comprise a hollow space 22 inwhich liquid nitrogen circulates, defined between the tubular shell 9and a supporting tubular element 23.

[0112] Externally to this non-coaxial monophase superconducting cable 1a layer of dielectric material 24 is provided for the electricalinsulation of the superconducting cable, incorporated within two tubularelements 25, 26 of semiconducting material.

[0113] With reference to what has been described hereinabove, someexamples will be provided hereunder which illustrate the behavior inshort circuit conditions as well as the mechanical stresses of someembodiments of the superconducting cables.

EXAMPLES 1-2 Invention

[0114] According to the invention, two prototypes of high power coaxialcable were made, comprising 3 conductive elements, each including a pairof phase and return conductors consisting of tapes of superconductingmaterial spirally wound on a respective supporting element, in this caseconsisting of a composite tubular element for the phase conductor and ofthe layer of dielectric material for the return conductor.

[0115] In particular, the composite supporting tubular element was madewith annular sectors made of copper (first metallic material) andpolytetrafluoroethylene, as polymeric material, alternately arranged oneafter the other.

[0116] In the case of Example 1, both the superconducting tapes of thephase conductor (coupled to the composite support) and thesuperconducting tapes of the return conductor (coupled to thestabilizing metal consisting of copper straps), were provided with areinforcing foil made of metal, coupled to the metallic coating of thetapes themselves, whereas in the case of Example 2, they were notprovided with such a reinforcing foil.

[0117] The coupling step of the reinforcing foil to the superconductingtapes was carried out by submitting, in a first step, the reinforcingfoil to a tensile stress in a substantially longitudinal direction andcoupling the same, in a second step, to the tapes in order to obtain aprestress of the superconducting material. In particular, the foil wassubmitted to a tensile stress of about 15.4*107 Pa (15.7 kg/mm²) thusobtaining a prestress degree of the superconducting material equal toabout 0.1%.

[0118] In the cable of Example 1, the reinforcing foil of the phaseconductor and the metallic material of the composite tubular elementwere electrically connected in a way known per se to the tapes ofsuperconducting material.

[0119] The working characteristics taken into consideration for themanufacture of the cables were the following: power 0.7 GVA nominalvoltage (phase-phase) 132 kV nominal current 3070 A critical current9210 A length 50 km

[0120] The cables were designed in such a way as to be stable at thefollowing short circuit conditions: short circuit current Icc 50 kAshort circuit duration Δtcc 0.5 s

[0121] further assuming:

[0122] 1) that the power dissipated during the short circuit transientis wholly converted into a temperature increase of the layer ofsuperconducting material, of the metallic coating that incorporates thesuperconducting material and of the metal in any way in electricalcontact therewith (supporting tubular element, metallic reinforcing foiland copper straps),

[0123] 2) that the dissipation is resistive with passage of all theshort circuit current through the metallic material in electricalconnection with the superconducting material,

[0124] 3) to limit the maximum temperature reached by thesuperconducting at the end of the short circuit well below the maximumallowable temperature Tamm, defined as the minimum temperature betweenthe critical temperature of the superconducting and the boilingtemperature of the cooling fluid, at the minimum working pressure,assuming that the temperature increase ΔT due to the short circuit isgiven by the following relation:

ΔTamm≦(Tamm−Tworking max)/f

[0125] wherein Tworking max is the maximum working temperature and f isthe safety coefficient.

[0126] A cable made with the aforesaid working characteristics has thefollowing working temperature and pressure ranges for the liquidnitrogen: minimum working temperature = 63.2 K maximum workingtemperature = 82 K maximum working pressure = 20 bar minimum workingpressure = 10 bar

[0127] Assuming that a BSCCO type high temperature superconductingmaterial is used, having a critical temperature of about 110 K, andsince she boiling temperature of the liquid nitrogen at 10 bar pressureis equal to 104 K, the maximum allowable temperature Tamm will coincidewith this value.

[0128] The determination of the amount of metallic material for ensuringthe cryostability of the cable in short circuit conditions was carriedout according to the following equation:

ΔTamm=[(ΣRiIcci ²)/(Σmicpi)]*Δtcc  (I)

[0129] wherein:

[0130] ΔTamm represents the allowable temperature increase due to shortcircuit,

[0131] Ri represents the resistance of the i-th element of thesuperconducting cable,

[0132] Icci represents the short circuit current of the i-th element ofthe superconducting cable,

[0133] mi represents the mass of the i-th element of the superconductingcable,

[0134] cpi represents the specific heat of the i-th element of thesuperconducting cable,

[0135] Δtcc represents the duration of the short circuit.

[0136] It results:

mi=δi*Vi=δi*Si*li  (II)

[0137] wherein:

[0138] δi represents the density of the i-th element,

[0139] Vi represents the volume of the i-th element,

[0140] Si represents the cross section of the i-th element,

[0141] li represents the length of the i-th element.

[0142] It also results:

Ri=ρi*(li/Si)  (III)

[0143] wherein ρi represents the specific electric resistivity of thei-th element.

[0144] Since the value of ΔTamm, δi, li, ρi, Icci, cpi, Δtcc, as well asthe cross sections of the superconducting tapes, of the metallicreinforcing foils and of the copper straps are known design data, bysubstituting the equation (II) and (III) in (I) it is possible todetermine of the cross section of the metallic material of thesupporting tubular element.

[0145] The structural characteristics of the two cable prototypes arereported in following Table I, particularly only with respect to thephase conductor. In a quite similar way, the structural characteristicsof the return conductor may be determined, in view of the fact that thesame short circuit current passes through both of them.

[0146] The sizes of the single sector were essentially chosen in orderto comply with installation and cooling constraints; in particular,sectors having an inner diameter equal to 38.7 mm and an outer diameterequal to 48.5 mm were chosen, for which a number of sectors equal to 14has been found to be appropriate.

[0147] The prototypes were then submitted to a number of tests thatallowed to evaluate the deformations generated in the superconductingmaterial, the traction force applied by the cable to the terminals as areaction to the constrained shrinkage.

[0148] The results of such tests are reported in the following Table II,wherein the value of the critical deformation is also reported, that isthe value of deformation above which a decrease of the current transportcapacity of the superconducting material, (probably due to fractures andgrain separation of the superconducting material) was detected.

[0149] In the aforesaid Table II the amount of conducting material usedwith respect to that required to ensure the full and adiabatic stabilityof the cable according to the criterion of the invention is alsoreported.

EXAMPLE 3 Comparison

[0150] With the purpose of making a comparison, a cable was madecomprising of a plurality of conductive elements, each including a pairof phase and return conductors consisting of tapes of superconductingmaterial spirally wound on a respective supporting element, in this caseconsisting of a tubular element entirely made of metal for the phaseconductor and of the layer of dielectric material for the returnconductor.

[0151] The tubular element entirely made of metal consisted of metallicsectors, in particular sectors made of copper.

[0152] According to the procedure described in the preceding Examples1-2, the aforesaid cable was designed according to the criterion ofcryostability of the invention, in such a way as to be stable in theshort circuit conditions reported in the same.

[0153] The structural characteristics of the cable are reported in thefollowing Table I.

[0154] Sectors having an inner diameter equal to 38.7 mm and an outerdiameter equal to 45.7 mm were chosen. The preferred number of annularsectors for such arrangement has been found equal to 16.

[0155] In a quite similar way, the cable was submitted to the testsdescribed in the preceding Examples 1-2, with respect to thedetermination of the mechanical stresses induced in the cable and of thestresses induced by the cable onto the terminals. The results of suchtests are reported in the same Table II.

[0156] Furthermore, the amount of metallic material used with respect tothe minimum amount required for ensuring the stability in short circuitconditions for the aforesaid cable is also reported in Table II.

[0157] As to the cable geometry, it turns out that, having set the sameinner diameter equal to 38.7 mm for all the cables of the threepreceding examples for hydraulic reasons, the supporting elemententirely made of metal of Example 3, because of construction reasons,implies the use of a copper section equal to 138% of the copper sectionof the composite tubular elements of Examples 1 and 2.

EXAMPLE 4 Comparison

[0158] With the purpose of making a comparison, a cable was madeaccording to the same characteristics of preceding Example 3, except forthe supporting tubular element, which was constituted by a tubularelement made of polymeric material, in particular made ofpolytetrafluoroethylene.

[0159] The aforesaid tubular element entirely made of polymericmaterial, was compared with the prototypes according to the inventiononly in terms of mechanical stresses induced in the superconductingmaterial and at the terminals, since the cable, being essentially devoidof metallic material adapted to transmit a significant current quantity,is not cryostable in short circuit conditions.

[0160] From the results of Table II, it is clear that in both prototypesof Examples 1 and 2 the deformations which the superconducting materialis submitted to are substantially lower than the critical value, andfurther are also clearly lower than those detected for the prototype ofExample 3, as an additional proof of the effectiveness o the compositesupporting tubular element in reducing the magnitude of the stressesalong a longitudinal direction imparted to the superconducting material.

[0161] With respect to the value of critical deformation, it is thenimmediately clear from the values of Table II that the cable providedwith reinforcing foil, Examples 1, has a greater value than the cablesnot provided with such foil; this may be ascribed to the prestresseffect of the layer of superconducting material consequent to thecoupling step of the foil to the metallic coating of the tapes.

[0162] It is further possible to observe that, for the cable of Example3 the deformation of the superconducting material has a greater valuethan the critical one, and this would affect the capacity of thesuperconducting of transmitting current in superconductivity conditions.

[0163] With respect to the prototype of Example 3, the prototypes ofExample 1 and 2, furthermore, exert much lower traction forces on theterminals, and this to the advantage of the mechanical stability of thecable.

[0164] Although in terms of deformations on the conductors, of criticaldeformation and of traction forces on the terminals a comparison betweenthe cable prototypes of the invention and the cable of Example 4 wouldinduce to prefer the latter as it would ensure a greater currenttransmission capacity without incurring in problems related tomechanical stresses of the superconductors, from the data related to theamount of metallic material present in the supporting element andrequired for facing possible short circuit conditions, it may beinferred that a cable according to Example 4 is not adequate for use inconditions wherein a risk of short circuits exists (and no differentprotection means are provided). TABLE I metallic material of theinternal supporting tubular element SC ΔT. (° C.) Ex. 1 sect. mm2 33588.6 10 Cu/SC 3.8 Ex. 2 sect. mm2 335 88.6 10 Cu/SC 3.8 Ex. 3 sect. mm2464 88.6 5.6 Cu/SC 5.2

[0165] TABLE II Ex. 1 Ex. 2 Ex. 3 Ex. 4 deformations on 0.18 0.18 0.310.05 superconductors (%) critical deformation (%) 0.5 0.29 0.29 0.29traction forces (Kg) 12500 12000 18600 5400 copper amount (%) 100 100138 —

1. Superconducting cable (1) comprising: a) a layer (20) of tapescomprising superconducting material, b) a tubular element (6) forsupporting said layer (20) of tapes comprising superconducting material,c) a cooling circuit, adapted to cool the superconducting material to aworking temperature not higher than its critical temperature,characterized in that said tubular element (6) is composite andcomprises a predetermined amount of a first material having a firstthermal expansion coefficient and a second material having a thermalexpansion coefficient higher than that of said first material, saidthermal expansion coefficients and said amounts of said first and secondmaterial being predetermined in such a way that said tubular element hasan overall thermal shrinkage between the room temperature and saidworking temperature of the cable such as to cause a deformation of saidtapes comprising superconducting material lower than the criticaldeformation of the same tapes.
 2. Superconducting cable (1) according toclaim 1, characterized in that said first and second material are formedas adjacent annular sectors (16, 17).
 3. Superconducting cable (1)according to claim 2, characterized in that said annular sectors (16,17) are circumferentially arranged one after the other. 4.Superconducting cable (1) according to claim 2, characterized in thatsaid annular sectors (16, 17) are spirally wound according to a windingangle comprised between 5° and 50°.
 5. Superconducting cable (1)according to claim 1, characterized in that said first material is ametal having a resistivity at 77 K<5*10-9 Ωm, a specific heat at 77K>106 J/m³K and a heat conductivity at 77 K>5 W/mK.
 6. Superconductingcable (1) according to claim 1, characterized in that said secondmaterial is a non metallic material having a thermal expansioncoefficient higher than 17*10-6° C.-1.
 7. Superconducting cable (1)according to claim 6, characterized in that said second non metallicmaterial is a polymeric material selected from the group comprising:polyamide, polytetrafluoroethylene and polyethylene.
 8. Superconductingcable (1) according to claim 1, characterized in that thesuperconducting material is incorporated within a metallic coating (19).9. Superconducting cable (1) according to claim 8, characterized in thatit comprises a plurality of superconducting tapes (18 a, 18 b) spirallywound on the surface of said at least one supporting tubular element (6)according to winding angles comprised between 5° and 60°. 10.Superconducting cable (1) according to claim 8, characterized in that itfurther comprises at least one reinforcing foil (21) made of metallicmaterial coupled to the metallic coating (19) of said superconductingmaterial, said foil (21) being in electrical connection with thesuperconducting material.
 11. Superconducting cable (1) according toclaim 8, characterized in that it comprises two reinforcing foils (21)made of metallic material coupled to opposite faces of saidsuperconducting material.
 12. Superconducting cable (1) according toclaim 10 or 11, characterized in that said superconducting material isessentially prestressed along a longitudinal direction. 13.Superconducting cable (1) according to claim 12, characterized in thatsaid material has a prestress degree along a longitudinal direction (γ)comprised between 0.05 and 0.2%.
 14. Superconducting cable (1) accordingto claim 10 or 11, characterized in that the reinforcing foil (21) andthe metallic coating (19) of said superconducting tape (18) consist of ametal selected from the group comprising: copper, aluminum, silver,magnesium, nickel, bronze, stainless steel, beryllium, and alloysthereof.
 15. Superconducting element (3) comprising at least one layer(20) of superconducting material supported by a tubular element (6),characterized in that said tubular element (6) is essentially compositeand comprises a predetermined amount of a first metallic material inelectrical contact with the layer (20) of superconducting material andat least one second polymeric material associated to said firstmaterial.
 16. Superconducting element (3) according to claim 15,characterized in that said first and second material are formed asadjacent annular sectors (16, 17).
 17. Superconducting element (3)according to claim 16, characterized in that said annular sectors (16,17) are arranged one after the other.
 18. Superconducting element (3)according to claim 16, characterized in that said annular sectors (16,17) are spirally wound according to a winding angle comprised between 5°and 50°.
 19. Superconducting element (3) according to claim 15,characterized in that said first metallic material is a metal having aresistivity at 77 K<5*10-9 Ωm, a specific heat at 77 K>106 J/m³K and aheat conductivity at 77 K>5 W/mK.
 20. Superconducting element (3)according to claim 15, characterized in that said second material is anon metallic material having a thermal expansion coefficient higher than17*10-6° C.-1.
 21. Superconducting element (3) according to claim 20,characterized in that said second polymeric material is selected fromthe group comprising: polyamide, polytetrafluoroethylene andpolyethylene.
 22. Superconducting element (3) according to claim 15,characterized in that it comprises at least one superconducting tape (18a, 18 b) wherein said layer (20) of superconducting material isincorporated within a metallic coating (19) and at least one reinforcingfoil (21) made of metallic material coupled to said metallic coating(19).
 23. Method for limiting the tensile stresses along a longitudinaldirection imparted as consequence of cooling to opposite fixingterminals of a superconducting cable of the type with clamped heads,said cable comprising at least one layer (20) of superconductingmaterial, characterized in that a composite tubular element (6) isprovided in the cable for supporting the layer (20) of superconductingmaterial.